汶川和芦山地震之间地震空区综合研究进展

doi:10.6038/cjg2018M0254

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摘要

为了探索汶川与芦山地震之间的地震空区的深部结构，探讨该地区的物质运移方式及动力学模式，评价区域断层的地震活动性，本研究小组于2015年5月至今共开展了2期共约3年的地震观测.前期得到的主要结论包括：（1）地震空区两端应力状态差异显著，且存在沿断裂带的横向应力差；（2）接收函数分析发现一个平行于马尔康断裂的Moho面上隆，泊松比较高的条带状结构从地震空区延伸到龙日坝断裂，可能是下地壳撕裂和部分熔融的结果；（3）地震空区以下，上地壳速度相对偏低，在下地壳存在明显的部分熔融体，这可能是地震能量不能积累的原因之一；（4）地震空区完整性震级M_c=0.2以上的地震的活动性仍然很低；基于密集的监测台阵（约70台地震计）的走时定位也初步显示了龙门山断裂带的地震空区段仍然很平静，但是其东边的断裂带（包括大邑断裂等以及一些隐伏断裂）上地震活动比较活跃.判断这些断裂可能是该区域应力积累的主要断裂，值得进一步关注；（5）提出了地震空区产生的机制：空区北东—南西两端水平应力差导致空区地壳的撕裂，地幔物质上涌，产生部分熔融，空区地壳抬升以塑性形变为主，穿过空区段的龙门山断裂带地震活动性低，地表地形较平滑.

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	梁春涛
	黄焱羚
	王朝亮
	刘志强
	杨宜海
	吴晶
	何富君

关键词 ：地震空区, 接收函数, 应力反演, 微震监测, 地震活动性

Abstract：

The purpose of this work is to investigate the deep structure beneath the seismic gap between the 2008 Wenchuan and 2013 Lushan earthquakes, discuss the geodynamic process of the movement of materials, and to assess the seismic potential in this region. We have conducted two phases of seismic observations with more than 100 temporary instruments deployed in the last three years. The preliminary results indicate that (1) the seismic gap serves as a transition zone separating two distinct stress regimes, and the fault-parallel stress difference is observed between the two sides of the gap. (2) The receiver function analysis shows that a long belt of Moho uplift extends from the gap to the Longriba fault, roughly aligning with the Barkam fault. The Poisson's ratio is relatively high along this belt. A partial melting at the lower crust along this belt may explain these observations. (3) Beneath the seismic gap, the velocity in the upper crust is relatively low and a thickened lower crust may indicate the existence of partial melting in the lower crust. These two observations may explain the absence of seismicity in the seismic gap. (4) Micro-seismicity was studied using a matched filter method. Given the complete magnitude of earthquakes being lowered from 1.4 to 0.2, the seismic gap is still void of events compared to the two sides of the gap; seismic locations are also obtained by traveltimes recorded by the densest array in the region deployed recently. The seismicity is still void of events, but a linear strip of seismicity was observed to the east of the seismic gap and it aligns roughly with the Dayi fault and/or other blind faults in the region. These faults could be the structure to yield big events in the future. (5) Based on the above observations, we propose a new model to account for the geodynamics of the seismic gap: the fault-parallel stress difference across the gap may imply that the gap is the point where the crust of the eastern Tibetan plateau is being torn apart, resulting in the upwelling of hot mantle materials to produce partial melting in the lower crust. This partial melting may heat up the upper crust beneath the gap and turn the faults in this segment into aseismic. The tearing apart fracture of the crust may extend all the way to the Barkam fault, but it less developed further away from the seismic gap in the Longmenshan fault system.

Key words： Seismic gap Receiver function Stress inversion Micro-seismicity

收稿日期: 2018-04-08

PACS:

P315

基金资助:

国家自然科学基金（41674059，41374058）资助.

作者简介: 梁春涛,E-mail:liangct@cdut.edu.cn

链接本文:

http://manu39.magtech.com.cn/Geophy/CN/10.6038/cjg2018M0254 或 http://manu39.magtech.com.cn/Geophy/CN/Y2018/V61/I5/1996

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